1. Field of the Invention
The present invention relates to fiber optic gyroscopes. More particularly, this invention pertains to apparatus for suppressing bias errors induced by magnetic fields oriented both transverse and axially with respect to the gyroscope sensor coil.
2. Description of the Prior Art
Fiber optic rotation sensing devices, such as gyroscopes, comprise two main components, (1) a front end including a light source and detector and (2) a fiber optic interferometer, including sensor coil, coupler and polarizer that are mounted to a system. Light from the source is split by the coupler into two beams, each of which is coupled into an opposed lead of the sensing coil. The interferometer and associated electronics process the phase relationship between the two interfering, counter-propagating beams of light when they emerge from opposite leads of the coil and are combined. A phase shift difference between the two beams results from (1) coil rotation and (2) so-called "environmental" factors.
Environmental factors include such variables as temperature, vibration (both acoustical and mechanical) and magnetic fields (Faraday effects). These factors can induce phase shifts between the counter-propagating beams that are indistinguishable from those induced by rotation. In the event that the sensing loop is of ideal single mode fiber, the Faraday effect is cancelled when the light travels through the fiber coil and a phase difference between the counterpropagating beams is not generated. The phase difference is observed, due to the nonreciprocity of the Faraday effect, when retarders are located asymmetrically within the fiber loop. Fiber twist, occurring naturally during manufacture or induced during the winding of the coil, acts as an actual and inevitable retarder that leads to bias drift in the presence of a magnetic field. One common method for avoiding the influence of magnetic fields is to place the sensor coil in a .mu.-metal housing. This solution is affected at the cost of an increase in both the weight and cost of the fiber optic gyro.
The Faraday effect in fiber loops is discussed in articles by Kazuo Hotate and Kunio Tabe ("Drift of an Optical Fiber Gyroscope Caused by the Faraday Effect: Influence of the Earth's Magnetic Field," Applied Optics, Vol. 25 No. 7 (Apr. 1, 1987) pp. 1086-1092 and "Drift of an Optical Fiber Gyroscope Caused by the Faraday Effect: Experiment," Journal of Lightwave Technology, Vol. LT-5, No. 7 (July 1987) pp. 997-1001). Hotate and Tabe discuss a relationship between the bias and drift of the fiber optic gyro (FOG) due to transversely-directed magnetic fields (i.e. fields substantially in the plane of the loops that, in combination, constitute the sensor coil) and the twisting of the optical fiber. Twisting of the polarization maintaining (PM) optical fiber is unavoidable as mentioned earlier, occurring during various stages of coil construction. Fiber fabrication inevitably imparts some twists. When the spool is then wound from the fiber, the nearly impossible-to-avoid misalignment of the coil winder and the gyro spool axis will produce further twisting. When the axis of the winding machine is at a tilt with respect to the axis of the fiber coil, twist is induced in the coil which is periodic with a twist rate that varies as a sinusoid as the fiber is wrapped about the circumference of the spool Angular misalignments on the order of milliradians can produce magnetic sensitivities on the order of degrees/hour-Gauss. While a large number of twist modes will be generated and randomly distributed within a resulting sensor coil, Hotate and Tabe have found and experimentally verified that (only) the twist component whose twist rate period is equal in fiber length to a loop of the sensor coil is responsible for the sensitivity to transverse magnetic fields.
The above-cited articles are confined to the effect of transverse magnetic fields and, thus, the authors' insights are of limited practical significance. In the real world, both transverse and axial magnetic field components are generally encountered. Hotate et al. suggest that one employ a polarization-maintaining fiber (PM fiber) sensor coil to suppress magnetic field sensitivity. In practice, however, the birefringence of currently-available PM fiber is not sufficiently large to suppress the bias error due to the Faraday effects completely. Bias errors of between 1 and 5 degree/hour-Gauss are normally detected in the output of a FOG having a PM fiber coil.